Significant developments in medical diagnostics have been brought about in recent years by the introduction of non-invasive methodics. Among these, near-infrared (I.R.) spectroscopy and instruments have been utilized to characterize biological tissues in vivo.
I. R. spectrophotometry rests upon the relative transparency of biological materials to photons in the near I. R. (700-900 nm). In situ photon transmission through organs is sufficient to permit monitoring of absorptive changes in the tissues. In this spectral region, only some chromophores of great functional significance absorb light: the heme of hemoglobin whereby changes in local hematic volume and equilibrium between oxyhemoglobin (HbO.sub.2) and hemoglobin (Hb) can be assessed, and the visible copper of cytochromeoxidase (cyt a,a.sub.3), i.e. the terminal enzyme in the mitochondrial respiratory chain which catalyzes 95% of the cell oxygen (O.sub.2) input.
Since the mitrochondrial respiratory chain is the main gateway to utilizing the free energy obtained in the various metabolisms, in vivo evaluation of the redox state of cyt a,a.sub.3 may be of great assistance in assessing the functional state of cells in various physiopathological situations (E. Dora, J. Neurochem. 42, 101-108, 1984; M. Erecinska, D. Wilson, J. Memb. Biology 70, 1-14, 1982; E. F. Jobsis, Adv. Neurol. 26,299, 1979).
Fairly accurate methods are known of measuring the level of oxygenation in the hemoglobin circulating through the vascular system of surface tissues (Takatani et al., Ann. Biomed. Eng. 8,1 1980). In general, however, such prior methods fail to provide quantitative results with internal organs owing to the difficulty encountered in evaluating the effects of light diffusion.
Jobsis, of Duke University, recently proposed to use this type of spectroscopy to characterize cell metabolism in vivo (U.S. Pat. No. 4,281,645), and in particular, to assess the oxygenation level of cerebral tissues by measuring the I.R. absorption of cytomchrome-c-oxidase (F. F. Jobsis, Science, 198,1264, 1977).
The spectrophotometer proposed by Jobsis comprises: (A) some light sources which emit sequentially radiation within the range of 700 to 1,300 nm; (B) a fiber optic which transmits the light to an organ to be monitored; (C) an optical fiber which picks up the emerging radiation from the monitored organ; and (D) a system for converting the radiation to a readily analyzed signal.
However, the spectrophotometer proposed by Jobsis provides unacceptable quantitative results because it takes into no account the effects of light diffusion, which are quite significant and may vary over time; further, and more specifically, light diffusion makes the optical path non-rectilinear and Beer-Lambert law does not apply.
Thus, the instrument is unable to correct the observed data due to scattering effects.